(1) Field of the Invention
The present invention relates to a semiconductor device that includes a gallium nitride compound semiconductor layer, and in particular to an improvement in the material of a Schottky contact used in the semiconductor device.
(2) Related Art
With broader-band wireless communication in recent years, demands have heightened for circuits that are able to operate at high frequency. Semiconductor components in such high-frequency circuits must also be compatible with high frequencies. Gallium nitride compound semiconductor components are showing promise as high-frequency power device materials that respond to the demands.
A gallium nitride compound semiconductor refers to a semiconductor that is a compound of gallium nitride (GaN), aluminum gallium nitride (AlGaN), indium gallium nitride (InGaN), indium aluminum gallium nitride (InAlGaN), or the like. Generally, such a compound is expressed as InXAlYGa1−X−YN (0≦X<1, 0≦Y<1, 0≦X+Y<1).
A gallium nitride compound semiconductor has high dielectric breakdown field strength, high thermal conductivity and high electron saturation velocity. Above all, a gallium nitride compound semiconductor of an AlGaN/GaN heterojunction structure has a field strength of 1*105 V/cm, and an electron velocity that is twice that of a gallium arsenic compound semiconductor. For this reason, expectations are high that superior high frequency characteristics will be realized if finer gallium nitride compound semiconductors are able to be manufactured.
One type of circuit component that uses this kind of gallium nitride compound semiconductor is a metal semiconductor field effect transistor (MESFET). An MESFET is a field effect transistor (FET) that includes a gallium nitride compound semiconductor that is doped with an n-type dopant such as silicon or germanium and exhibits n-type characteristics, and on which a Schottky contact is arranged as a gate contact. Ohmic contacts are used as the source contact and the drain contact.
In order to have the field effect transistor operate at high frequency, it is necessary to shorten the length of the gate contacts, and lower the capacitance of the gate contacts themselves. In particular, in a device in which favorable high frequency characteristics are required, it is essential to have a fine gate of a sub-half-micron or less in length. However, when the gate length is shortened, adhesion between the gate contact and the gallium nitride compound semiconductor is poor, causing the Schottky contact to peel in the semiconductor process or to come off the substrate. This results in a problem of a reduced yield ratio.
Furthermore, generally a metal such as gold, platinum, palladium or nickel is used as the Schottky contact. However, the better the Schottky characteristics (such as barrier height and ideality factor n) of the metal material is, the more easily the Schottky contact peels.
For this reason, realization of superior Schottky characteristics with high frequency is a difficult technical problem.
Note that barrier height Φb and ideality factor n are defined in Dieter K. Schroder, “Semiconductor Material and Device Characterization, Second Edition”, John Wiley & Sons, Ltd., pp.168-173, July 1998.